The fabrication of these micro-machined sensors, also called MEMS (Micro-Electro-Mechanical-Systems) sensors, uses techniques of collective micro-machining, etching, doping depositions, etc., similar to those which are used for the fabrication of electronic integrated circuits, allowing low production costs.
Such MEMS inertial sensors produced on a silicon or quartz wafer by micro-machining are already known. The structure is planar, in the plane of the silicon or quartz substrate in which it is etched.
These sensors consist of several vibrating mobile masses linked together and to their support by elastic elements in such a way as to constitute an excitation resonator, or primary resonator, and a detection resonator, or secondary resonator, the two resonators being coupled together by the Coriolis acceleration. These sensors have means of excitation, of detection, and often of balancing. In these sensors, the masses are generally excited in vibration in the plane XY of the wafer, perpendicularly to an axis Z, termed the “sensitive axis” of the gyrometer. When the gyrometer rotates about its sensitive axis, the composition of the forced vibration with the angular rotation vector produces, through the Coriolis effect, forces which set the mobile masses into natural vibration perpendicularly to the excitation vibration and to the sensitive axis; the amplitude of this natural vibration is proportional to the rotation speed. The natural vibration is detected by a detection transducer, whose electrical signals are utilized by an electronic circuit to deduce therefrom a value of the angular speed about the sensitive axis.
Various structures of MEMS vibrating inertial sensors are known which define the shapes and dispositions of the various elements of the structure. These various elements will typically be the vibrating elements, the mechanisms for suspending these elements, the coupling mechanisms; the excitation and detection electrostatic transducers allowing actual measurement and the electrostatic transducers allowing various balancings or compensations making it possible to improve the precision of the measurement.
These structures are devised to satisfy various constraints with regard to measurement precision and to low energy losses, while remaining within the domain of MEMS fabrication technologies.
The performance of such sensors may be degraded by the energy losses of the resonators to the exterior. To limit these energy losses, the excitation resonator of most gyrometers is balanced to first order by the use of two masses vibrating in phase opposition, like a tuning fork. The useful vibration mode in phase opposition is separated from the parasitic in-phase mode by virtue of a central elastic coupling element which introduces a stiffness between the two masses. An example of such a sensor is described in patent FR2846740. However, energy losses persist in such sensors, since the secondary resonator is not balanced by construction. Therefore, this mode transmits a torque to the support of the tuning fork, thereby rendering this mode sensitive to the conditions of fixing to the support and to the exterior disturbances transmitted by the support.
Tuning fork gyrometer structures comprising elastic means of coupling between the branches of the tuning fork are well known and described in the patent literature. In these structures, the vibrations of the masses in phase opposition are therefore utilized by separating the useful vibration mode from the parasitic modes. To improve the measurement precision, these structures propose various adjustment or compensation means, be it by construction and/or through the use of electrically controlled compensation or adjustment elements.
Structures with double tuning fork, using four masses, have also been proposed so as to compensate by construction the defects of balancing of the secondary resonator of the single tuning fork structure. Patents FR2859527 and FR2859528 give examples thereof.